Hello, I'm Dr. Kumar Swamy, and I'll be talking about myokymia. Myokymia is the name given to the clinical phenomenon of quivering, undulating, or rippling muscle movements that are involuntary. The electromyographic correlate of myokymia is what are known as myokymic discharges. These are grouped, rhythmic, spontaneous discharges that repeatedly arise from the same motor unit. They have a fairly fast firing frequency of between 5 and 60 hertz within a burst, and a little slower at less than 2 hertz between successive bursts of the same motor unit. They're usually associated with other discharges that also arise from individual motor units, such as doublets, triplets, and repeated vesiculations, which in this case are called singlets. They are thought to originate from apheptic transmission across demyelinated nerve membranes or from spontaneous depolarization of axons. The presence of myokymic discharges on needle EMG can be quite helpful. For instance, in patients with plexopathy and a history of cancer, myokymic discharges in the affected limb indicate radiation injury as the likely cause of the plexopathy rather than cancer infiltration, and this is a clinically important distinction. But these discharges can also be seen in a variety of other conditions. For instance, facial myokymia is more common than limb myokymia and can occur in multiple sclerosis and with pontine lesions such as gliomas. Myokymic discharges can also be seen in compressive and autoimmune demyelinating neuropathies, radiculopathies, disorders of motor neurons, as a transient feature of rattlesnake envenomation, and as a generalized finding in hypocalcemic tetanus. They may also be seen in association with neuromyotonic discharges as part of a peripheral nerve hyperexcitability syndrome such as Isaac syndrome. Myokymic discharges may share some features with other discharges arising from individual motor units as depicted in this chart, but from a diagnostic perspective, it's probably more important to distinguish them from other types of activity such as tremor, poorly sustained complex repetitive discharges, and brief myotonic discharges that sometimes lack the typical waxing and waning frequency and amplitude. Most of these distinctions are easily made by freezing the recorded waveforms and reviewing them with a lower sweep speed or a longer time base. Let's look at the example of a patient who was referred to the EMG lab with these involuntary movements of the right hand. We see these repetitive almost twitchy movements of the fingers and the more undulating type movements of the thenar and hypothenar eminence. Needle EMG of the first dorsal introspeus showed these findings. So that's the characteristic marching, bursting sound of myokymia. Looking closely at the waveforms from that recording, we see that the same motor unit was firing repeatedly with a maximum frequency of about 15 Hz within a burst and a frequency of around 0.2 to 0.3 Hz between bursts arising from the same motor unit. We also see these waveforms accompanying the myokymic discharges, again made up of the same motor unit firing twice here. So these are two doublets arising obviously from two different motor units. Here's another recording from her abductor digiti minimi muscle. You'll notice that the myokymia in this muscle was more subtle and better heard than seen. In addition to the myokymia, which is rather distant and in the background, we see this triplet, a doublet here, and a singlet, which were firing repeatedly. These are my references and thank you for listening.

myokymia

involuntary muscle movements

myokymic discharges

neurological conditions

needle EMG recordings